1. Field of Invention
The invention relates to a process for pre-register setting in a printing machine with multiple printing units.
2. Description of the Prior Art
For multicolor printing, printing machines—and here in particular, gravure printing machines—apply individual colors in individual printing units. In the printing process, the positions of the images with the different colors with respect to each other are checked by means of register controls. For that purpose, the register marks are in general recorded and their distances are kept constant.
This type of register methods are discussed, among others, in the hitherto unpublished German patent application with the application number 102 54 836. Before the printing process starts, and before these controls are used, the printed images must be mutually so aligned that the register marks lie within the capture area of the used measuring system of the regulator (pre-register setting). For that, the phasing of the cylinder and/or the web length between the printing units is adjusted. The effective sheet length of the webs between the printing units is in general not an integral multiple of the printing length.
As a rule, the pre-registering takes place during the preparation of the press. The printing length varies according to the motif and the anticipated web extensions. The effective web length between the printing units varies due to the selected path of the web, due to the different sizes of the printing cylinders and the impression roller, due to the mechanical flexibility and the tolerances, due to process parameters like the web tension and the drying temperature, as well as due to physical properties of the printing material, such as its modulus of elasticity and its geometrical dimensions.
The pre-register setting is done according to the state of the art for the printing job, for which the preparations are done for the first time, in general manually. The print images are printed at random positions on the web. In stationary machines, the position deviation is then determined through length measures. This data is entered as correction values in the machine regulation, which determines and sets the phase displacement based on it. In a reapplied contact pressure, the printed images then overlap to a large extent. The required time and the quantity of the discards are a disadvantage in this method.
Automated pre-register settings require entry of the web length as the basis for the calculations for the phase of the cylinder in the preparations for the first time. However, the effective web length varies much, especially in case of flexible printing substrates, due to the variations in the length of the material, so that the capture area is in general missed and the process is therefore employed only seldom. Thereby, the tolerance in the alignment of the printing cylinder with the rotation angle receiver of the drive also proves to be problematic.
A pre-register setting for a repetitive application can be based on stored information about the phasing of the printing cylinder for the original application. However, this process requires otherwise also exact repetition of all process and machine parameters (even the ambient temperature and the humidity) and has consequently been less successful in general.